This invention relates generally to a computer-controlled object-building system and, in particular, to an improved solid freeform fabrication system for building a three-dimensional object such as a model or a molding tool.
Solid freeform fabrication (SFF) or layer manufacturing is a new rapid prototyping and tooling technology. A SFF system builds an object layer by layer or point by point under the control of a computer. The process begins with creating a Computer-Aided Design (CAD) file to represent the desired object. This CAD file is converted to a suitable format, e.g. stereo lithography (.STL) format, and further sliced into a large number of thin layers with the contours of each layer being defined by a plurality of line segments connected to form vectors or polylines. The layer data are converted to tool path data normally in terms of computer numerical control (CNC) codes such as G-codes and M-codes. These codes are then utilized to drive a fabrication tool for building an object layer by layer.
The SFF technology has found a broad range of applications such as verifying CAD database, evaluating design feasibility, testing part functionality, assessing aesthetics, checking ergonomics of design, aiding in tool and fixture design, creating conceptual models and sales/marketing tools, generating patterns for investment casting, reducing or eliminating engineering changes in production, and providing small production runs. Although most of the prior-art SFF techniques are capable of making 3-D form models of relatively weak strength, few are able to directly produce material processing tools (such as molds for injection molding) with adequate accuracy and good speed.
A commercially available system, fused deposition modeling (FDM) from Stratasys, Inc. (Minneapolis, Minn.), operates by employing a heated nozzle to melt and extrude out a nylon wire or wax rod. The nozzle is translated under the control of a computer system in accordance with previously sliced CAD data. The FDM technique was first disclosed in U.S. Pat. No. 5,121,329 (1992), issued to Crump. This process requires preparation of a raw material into a flexible filament or a rigid rod form and, in real practice, has met with difficulty in extruding high temperature metal or ceramic materials. A more recent patent (U.S. Pat. No. 5,738,817, April 1998, to Danforth, et al.) reveals a FDM process for forming 3-D objects from a mixture of a particulate composition dispersed in a binder. The binder is later burned off with the remaining particulate composition densified by either metal impregnation or high-temperature sintering. Other melt extrusion-type processes include those disclosed in Valavaara (U.S. Pat. No. 4,749,347, June 1988), Masters (U.S. Pat. No. 5,134,569, July 1992), and Batchelder, et al. (U.S. Pat. No. 5,402,351, 1995 and U.S. Pat. No. 5,303,141, 1994). These melt extrusion based deposition systems are known to provide a relatively poor part accuracy. For instance, a typical FDM system provides an extruded strand of 250 to 500 xcexcm, although a layer accuracy as low as 125 xcexcm is achievable. The accuracy of a melt extrusion rapid prototyping system is limited by the orifice size of the extrusion nozzle, which cannot be smaller than approximately 125 xcexcm in real practice. Otherwise, there would be excessively high flow resistance in an ultra-fine capillary channel.
In U.S. Pat. No. 4,665,492, issued May 12, 1987, Masters teaches part fabrication by spraying liquid resin droplets, a process commonly referred to as Ballistic Particle Modeling (BPM). The BPM process includes heating a supply of thermoplastic resin to above its melting point and pumping the liquid resin to a nozzle, which ejects small liquid droplets from different directions to deposit on a substrate. Patents related to the BPM technology can also be found in U.S. Pat. No. 5,216,616 (June 1993 to Masters), U.S. Pat. No. 5,555,176 (September 1996, Menhennett, et al.), and U.S. Pat. No. 5,257,657 (November 1993 to Gore). Sanders Prototype, Inc. (Merrimack, N.H.) provides inkjet print-head technology for making plastic or wax models. Multiple-inkjet based rapid prototyping systems for making wax or plastic models are available from 3D Systems, Inc. (Valencia, Calif.). Model making from curable resins using an inkjet print-head is disclosed by Yamane, et al. (U.S. Pat. No. 5,059,266, October 1991 and U.S. Pat. No. 5,140937, August 1992) and by Helinski (U.S. Pat. No. 5,136,515, August 1992). Inkjet printing involves ejecting fine polymer or wax droplets from a print-head nozzle that is either thermally activated or piezo-electrically activated. The droplet size typically lies between 30 and 50 xcexcm, but could go down to 13 xcexcm. This implies that inkjet printing offers a high part accuracy. However, building an object point-by-point with xe2x80x9cpointsxe2x80x9d or droplets as small as 13 xcexcm could mean a slow build rate.
In a series of U.S. Pat. (No. 5,204,055, April 1993, U.S. Pat. No. 5,340,656, August 1994, U.S. Pat. No. 5,387,380, February 1995, and U.S. Pat. No. 5,490,882, February 1996), Sachs, et al. disclose a 3-D printing technique that involves using an ink jet to spray a computer-defined pattern of liquid binder onto a layer of uniform-composition powder. The binder serves to bond together those powder particles on those areas defined by this pattern. Those powder particles in the xe2x80x9cnegativexe2x80x9d regions (without binder) remain loose or separated from one another and are removed at the end of the build process. Another layer of powder is spread over the preceding one, and the process is repeated. The xe2x80x9cgreenxe2x80x9d part made up of those bonded powder particles is separated from the loose powder when the process is completed. This procedure is followed by binder removal and metal melt impregnation or sintering. Again, ejection of fine liquid droplets to bond a large area of powder particles could mean a long layer-building time. Additionally, it is difficult to entirely remove the polymer binder material from the finished 3-D object. The presence of binder residue could reduce the strength and other desired properties of the object. The metal melt impregnation process results in a honey-comb type structure of less than desirable properties and subjects this structure to creeping or warping during sintering of the original host material. The selected laser sintering or SLS technique (e.g., U.S. Pat. No. 4,863,538) involves spreading a full-layer of powder particles and uses a computer-controlled, high-power laser to partially melt these particles at desired spots. Commonly used powders include thermoplastic particles or thermoplastic-coated metal and ceramic particles. The procedures are repeated for subsequent layers, one layer at a time, according to the CAD data of the sliced-part geometry. The loose powder particles in each layer are allowed to stay as part of a support structure. The sintering process does not always fully melt the powder, but allows molten material to bridge between particles. Commercially available systems based on SLS are known to have several drawbacks. One problem is that long times are required to heat up and cool down the material chamber after building. In addition, the resulting part has a porous structure and subsequent sintering or infiltration operations are needed to fully consolidate the part.
In U.S. Pat. No. 5,555,481 (September 1996) Rock and Gilman disclose a freeform powder molding (FPM) method. A first class material and a second class material are deposited on a surface wherein the first class material forms a 3-D shape defined by the interface between the first class material and the second class material. The first class material is unified by subsequent processing such as sintering or fusion-and-solidification, which is followed by removing the second class material from the 3-D part made up of first class material. The second class material plays the basic role of serving as a support structure. Upon completion of the deposition procedure for all layers, the green object which has been compacted but not yet unified is highly delicate and fragile, prone to shape changes during subsequent handling. The final unification procedure tends to involve dimensional or shape changes in a part, thereby compromising the part accuracy. For instance, sintering of ceramic or metallic particles is known in the field of powder technology to involve large shrinkage. Solidification of a crystalline material (polymer, metal, and ceramic) from the melt state to the solid state are normally attendant with a large volume change. Since these geometry changes are allowed to occur at the end of the part building process, it is extremely difficult to exercise any corrective action to ensure the part accuracy.
In U.S. Pat. No. 5,514,232 (May 1996), Burns discloses a method and apparatus for automatic fabrication of a 3-D object from individual layers of fabrication material having a predetermined configuration. Each layer of fabrication material is first deposited on a carrier substrate in a deposition station. The fabrication material along with the substrate are then transferred to a stacker station. At this stacker station the individual layers are stacked together, with successive layers being affixed to each other and the substrate being removed after affixation. One advantage of this method is that the deposition station may permit deposition of layers with variable colors or material compositions. In real practice, however, transferring a delicate, not fully consolidated layer from one station to another would tend to shift the layer position and distort the layer shape. The removal of individual layers from their substrate also tends to inflict changes in layer shape and position with respect to a previous layer, leading to inaccuracy in the resulting part. It is difficult to use this process for fabricating metal parts; additional fusion, sintering, or diffusion bonding treatments are required to affix layers together.
In U.S. Pat. No. 5,301,863 (Apr. 12, 1994), Prinz and Weiss disclose a Shape Deposition Manufacturing (SDM) system. The system contains a material deposition station and a plurality of processing stations (for mask making, heat treating, packaging, complementary material deposition, shot peening, cleaning, shaping, sand-blasting, and inspection). Each processing station performs a separate function such that when the functions are performed in series, a layer of an object is produced and is prepared for the deposition of the next layer. This system requires an article transfer apparatus, a robot arm, to repetitively move the object-supporting platform and any layers formed thereon out of the deposition station into one or more of the processing stations before returning to the deposition station for building the next layer. These additional operations in the processing stations tend to shift the relative position of the object with respect to the object platform. Further, the transfer apparatus may not precisely bring the object to its exact previous position. Hence, the subsequent layer may be deposited on an incorrect spot, thereby compromising part accuracy. The more processing stations that the growing object has to go through, the higher the chances are for the part accuracy to be lost. Such a complex and complicated process necessarily makes the over-all fabrication equipment bulky, heavy, expensive, and difficult to maintain. The equipment also requires attended operation. Earlier patents related to SDM include U.S. Pat. No. 5,126,529 (June 1992), U.S. Pat. No. 5,207,371 (May 1993), and U.S. Pat. No. 5,301,415 (April 1994), all issued to Prinz, et al.
There are other methods that also make use of the approach of combined layer-addition and layer-subtraction: e.g., U.S. Pat. No. 5,398,193, March 1995 to deAngelis. These methods use metal deposition in conjunction with a metal removal technique such as milling, grinding, and the like. The xe2x80x9cstaircase effectxe2x80x9d and the roughness at the edge of each layer are reduced or eliminated by machining each layer and its peripheries after the layer is deposited.
Methods that involve deposition of metal parts from a steam of liquid metal droplets are disclosed in Orme, et al (U.S. Pat. Nos. 5,171,360; 5,226,948; 5,259,593; and 5,340,090) and in Sterett, et al. (U.S. Pat. Nos. 5,617,911; 5,669,433; 5,718,951; and 5,746,844). The method of Orme, et al involves directing a stream of a liquid material onto a collector of the shape of the desired product. A time dependent modulated disturbance is applied to the stream to produce a liquid droplet stream with the droplets impinging upon the collector and solidifying into a unitary shape. The method of Sterett, et al entails providing a supply of liquid metal droplets with each droplet being endowed with a positive or negative charge. The steam of liquid droplets is focused by passing these charged droplets through an alignment means, e.g., an electric field, to deposit on a target in a predetermined pattern. The deflection of heavy droplets of liquid metal by an electric field is not easy to accomplish. Further, a continuous supply of liquid metal droplets may make it difficult to prevent droplets from reaching xe2x80x9cnegativexe2x80x9d regions (which are not portions of a cross-section of the object). A mask will have to be used to collect these unwanted droplets.
A welding-type SFF method is disclosed in U.S. Pat. No. 5,578,227 issued in November 1996 to Rabinovich. This method involves directing a laser beam toward a rectilinearly movable stage for fusing a rectangular wire to a substrate or a previously fused wire layer on the stage. The wire is driven from a spool through a nozzle to be laid flat near a focused spot of the laser beam. Movement of the movable stage is controlled by a computer and stepper motors to follow a predetermined pattern of a cross-section of the desired object. In the welding process described in U.S. Pat. No. 5,281,789 (January 1994 to Merz, et al), a molten metal is deposited on a work surface and subsequent layers of metal are deposited thereon. An electrode and a weld torch are moved as a unit so that the molten metal may be deposited onto selective locations on the work surface. The droplet size is controlled by applying additional mechanical energy to the feed wire to constantly vibrate the feed metal. This is not a reliable mechanism for precise control of the droplet size. Both methods, specified in ""227 and ""789, do not readily permit variations in material composition. The weld pool size in these processes is too large to provide adequate part accuracy.
Another welding-type SFF process was developed by the researchers at Sandia National Labs. This process, laser-engineered net shaping (LENS), can make 3-D metallic components directly from CAD models. The LENS process entails injecting metal powder into a pool of molten metal created by a focused laser beam as the substrate below is slowly moved to trace out the contours of an object layer-by-layer. This process has been utilized to fabricate parts from a variety of metals, including 316 stainless steel, H13 tool steel, Inconel 625, tungsten, Ti-6Al-4V, nickel aluminide, and others. This process melts metallic powders completely and produces a fully dense material that does not require post-fabrication operations such as sintering. Again, it is difficult to maintain a weld pool smaller than 0.02 inch (0.5 mm or 500 xcexcm) in diameter; thus resulting in a Z-axis accuracy of no better than 250-350 xcexcm.
Most of the prior-art layer manufacturing techniques have been largely limited to producing parts with homogeneous material compositions. Furthermore, due to the specific solidification mechanisms employed, many other techniques are limited to producing parts from specific polymers. For instance, Stereo Lithography and Solid Ground Curing (SGC) rely on ultraviolet (UV) light induced curing of photo-curable polymers such as acrylate and epoxy resins. Polymers do not have adequate strength and thermal stability for use as tooling materials. As indicated earlier, droplet deposition, melt extrusion, or welding-type method alone does not meet the two critical requirements imposed upon a rapid prototyping (RP) or rapid tooling (RT) system: speed and accuracy. For instance, liquid droplet ejection features high accuracy but low speed, while melt extrusion or welding features relatively higher speed but much lower accuracy. Additionally, all layer-additive techniques produce parts with a xe2x80x9cstaircasexe2x80x9d appearance (see FIG.6a), which compromises the part accuracy.
Therefore, an object of the present invention is to provide an improved layer-additive process and apparatus for producing an object with high build rate and part accuracy.
Another object of the present invention is to provide a computer-controlled process and apparatus for producing a multi-material 3-D object on a layer-by-layer basis.
It is a further object of this invention to provide a computer-controlled object-building process that does not require heavy and expensive equipment.
It is another object of this invention to provide a process and apparatus for building a CAD-defined object in which the material composition distribution can be predetermined.
Still another object of this invention is to provide a layer manufacturing technique that places minimal constraint on the range of materials that can be used in the fabrication of a 3-D object.
The Process
The objects of the invention are realized by a process and related apparatus for fabricating a three-dimensional object on a layer-by-layer basis. Basically, the process comprises providing a focused heat source to maintain a small pool of molten material on the surface of a movable stage. The material in this pool is replenished, continuously or intermittently, by injecting metal and/or ceramic powder into this pool. The stage is controlled to move relative to the heat source to trace out the geometry of a bulky portion of a first layer for the desired object. The xe2x80x9cscanningxe2x80x9d of this pool (heat source-powder interaction zone) leaves behind a strand of molten material which substantially solidifies immediately after the material moves out of the heat-affected zone. Other portions of an object, particularly those containing fine features of a layer, are built by ejecting and depositing fine liquid droplets for improved accuracy. These two procedures are repeated concurrently or sequentially under the control of a CAD computer to deposit consecutive layers in sequence, thereby forming the desired 3-D object. The preferred heat source is a laser beam. Both liquid droplets and melt pool material compositions can be selected from a wide range of materials. Preferably, fine droplets of solidifiable liquid compositions are deposited to form a gradient-thickness zone to reduce or eliminate the staircase effect near any exterior peripheral zone.
Specifically, in one embodiment, the process comprises the steps of:
(a) operating a material deposition sub-system above a work surface with this sub-system comprising (1) a multiple-channel droplet deposition device for supplying multiple liquid compositions and ejecting droplets of selected liquid compositions on demand, (2) a focused heat source such as a laser beam that produces a heat-affected zone on the work surface, and (3) a powder dispensing device to inject fine solid particles into the heat-affected zone for maintaining a pool of molten material on the work surface;
(b) during the droplet ejecting and powder dispensing process, moving the material deposition sub-system and the work surface relative to one another in an X-Y plane defined by first (X-) and second (Y-) directions and in a third or Z-direction orthogonal to the X-Y plane to form the liquid droplets and melt pool material (collectively referred to as deposition materials) into a three dimensional object. Preferably, the weld pool is used to build the bulk of the object while the fine ejected droplets are used to build peripheries to reduce the xe2x80x9cstaircase effectxe2x80x9d or other regions containing fine features. Different liquid channels may supply different liquid compositions; e.g., different types of material and/or different additives. The powder dispensing device preferably is also capable of supplying powder particles of variable compositions.
Preferably, the above-cited moving step includes the steps of (i) moving the deposition sub-system and the work surface relative to each other in a direction parallel to the X-Y plane to form a first layer of the deposition materials on the work surface; (ii) moving the deposition subsystem and the work surface away from one another by a predetermined layer thickness; and (iii) after the portion of the first layer adjacent to the nozzles of the deposition sub-system has substantially solidified, dispensing a second layer of the deposition materials onto the first layer while simultaneously moving the work surface and the deposition sub-system relative to each other in a direction parallel to the X-Y plane, whereby the deposition materials in the second layer solidify and adhere to the first layer.
Preferably, the above steps are followed by additional steps of forming multiple layers of the deposition materials on top of one another by repeated dispensing of the liquid droplets and powder particles from the deposition devices as the work surface and the deposition sub-system are moved relative to each other in a direction parallel to the X-Y plane, with the deposition sub-system and the work surface being moved away from one another in the Z-direction by a predetermined layer thickness after each preceding layer has been formed, and with the deposition of each successive layer being controlled to take place after the deposition materials in the preceding layer immediately adjacent the deposition sub-system have substantially solidified.
Further preferably, the above cited steps are executed under the control of a computer by taking the following procedures: (f) creating an image of the three-dimensional object on a computer with the image including a plurality of segments defining the object; (g) generating programmed signals corresponding to each of the segments in a predetermined sequence; and (h) moving the deposition sub-system and the work surface relative to each other in response to the programmed signals. To build a multi-material object, each segment is preferably attached with a material composition code that can be converted to programmed signals for activating the ejection of selected liquid compositions and dispensing selected powder compositions to form a desired material distribution of the finished object. Further preferably, the supporting software programs in the computer comprise means for evaluating the CAD data files of the object to locate any un-supported feature of the object and means for defining a support structure for the un-supported feature. The software is also capable of creating a plurality of segments defining the support structure and generating programmed signals required by a deposition channel to fabricate the support structure.
The above-cited multiple-channel liquid droplet deposition device may simply be embodied by a plurality of separate droplet deposition devices with each device being supplied with possibly different liquid compositions and being capable of ejecting the liquid compositions in the form of droplets on demand. At least one device or channel is employed to deposit droplets of an object-building material while a second device is used for depositing a support structure.
The Apparatus
One embodiment of this invention is an apparatus comprising a material deposition sub-system, a work surface, and motion devices. The material deposition sub-system is composed of three major components: a liquid droplet deposition device, a focused heat source (a laser beam, e.g.), and a powder-dispensing device. The liquid droplet deposition device comprises (1) a multiplicity of flow channels with each channel being supplied with a solidifiable liquid composition, (2) at least one nozzle having a fluid passage in flow communication with one corresponding channel and a discharge orifice, and (3) actuator means located in control relation to these channels for activating droplet ejection through these discharge orifices. The focused heat source, disposed in working proximity to the work surface, creates a small heat-affected zone on the work surface. The powder-dispensing device comprises (1) at least a flow channel being supplied with solid powder particles, (2) a nozzle having a flow passage in flow communication with the flow channel and a discharge orifice, and (3) valve means located in control relation with the corresponding flow channel. The discharge orifice is disposed above the work surface but in the vicinity of the heat affected zone in such a fashion that the powder particles are discharged to this zone for forming a molten material pool of a predetermined size. The valve means operate to supply a metered amount of powders of predetermined material compositions, continuously or intermittently, to maintain a substantially constant-sized pool of molten material.
The work surface is generally flat and is located in close, working proximity to the discharge orifices of the deposition sub-system to receive discharged materials therefrom. The motion devices are coupled to the work surface and the material deposition sub-system for moving the deposition sub-system and the work surface relative to one another in the X-Y plane and in the Z-direction. Preferably, the liquid droplet device and the powder dispensing device are attached together to move as an integral unit. The motion devices are preferably controlled by a computer system for positioning the deposition sub-system with respect to the work surface in accordance with a CAD-generated data file representing the object. Further preferably, the same computer is used to regulate the operations of the material deposition sub-system in such a fashion that liquid droplets and powder particles are dispensed in predetermined sequences and at predetermined proportions.
The work surface is preferably provided with a controlled atmosphere wherein the temperature, pressure (including vacuum conditions), and gas composition can be regulated to facilitate solidification and to protect against possible metal oxidation. Preferably, sensor means are provided to periodically measure the dimensions of an object being built and send the acquired dimension data to the CAD computer so that new sets of logical layers may be re-calculated when necessary.
The droplet deposition device may be made to be similar to a multi-channel print-head commonly used in an ink jet printer. The print-head is equipped with heating means to maintain the material in a liquid state. Ink jet print-heads can generally be divided into two types: one type using thermal energy to produce a vapor bubble in an ink-filled channel that expels a drop of liquid while a second type using a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle. Droplets are dispensed through an orifice to deposit onto predetermined regions of a surface upon which a layer is being built.
Advantages of the Invention
The process and apparatus of this invention have several features, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention as expressed by the claims which follow, its more prominent features will now be discussed briefly.
After considering this brief discussion, and particularly after reading the section entitled xe2x80x9cDESCRIPTION OF THE PREFERRED EMBODIMENTSxe2x80x9d one will understand how the features of this invention offer its advantages, which include:
(1) The present invention provides a unique and novel process for producing a three-dimensional object on a layer-by-layer basis under the control of a computer. Both speed and accuracy, which are normally considered to be mutually exclusive in a prior-art layer manufacturing technique, can be achieved with the present method.
(2) Most of the prior-art layer manufacturing methods, including powder-based techniques such as 3-D printing (3DP) and selective laser sintering (SLS), are normally limited to the fabrication of an object with a uniform material composition. In contrast, the presently invented process readily allows the fabrication of an object having a spatially controlled material composition comprising two or more distinct types of material. For example, functionally gradient components can be readily fabricated with the present method.
(3) The presently invented method provides a layer-additive process which places minimal constraint on the variety of materials that can be processed. The liquid composition and the solid powder may be selected from a broad array of materials.
(4) The present method provides an adaptive layer-slicing approach and dimension sensor means to allow for in-process correction of any layer thickness variation. The present invention, therefore, offers a preferred method of layer manufacturing when part accuracy is a desirable feature.
(5) The present invention makes it possible to produce fully dense metal parts directly from the final design materials, thus eliminating intermediate or secondary processing steps sych as final sintering or metal impregnation required of 3DP and SLS. This feature enables this new technology to offer dramatic reductions in the time and cost required to realize functional metal parts. Hence, the technology provides both rapid prototyping and rapid tooling capabilities.
(6) The method can be embodied using simple and inexpensive mechanisms, so that the fabricator equipment can be relatively small, light, inexpensive and easy to maintain.